Embodiments of the present invention relate to an energy storage system, in particular to an energy storage system arranged to operate at elevated temperatures.
Wellbores are generally drilled to extract natural resources such as crude oil and gas. Downhole electrical equipment is used in wellbores for various purposes, such as for sensors to measure downhole conditions, to power data storage and transmission functions and to power downhole tools and valves. Such downhole electrical equipment can be powered by energy storage devices such as batteries, fuel cells or capacitors.
Many types of energy storage devices, such as NaCl batteries and fuel cells, only operate at higher temperatures. However, the operating temperatures range for many energy storage devices is above even the temperatures encountered downhole, which may, for example be 180° C. or above.
U.S. Pat. No. 4,416,000 discloses an arrangement to “start up” a battery before it is immersed into a hot environment of an oil well. Initially an external voltage is applied to the instrument while at the surface before immersion into the oil well. This voltage activates heating elements which melt the electrolyte of a molten salt battery. A thermostatic switch is provided so that when downhole, the battery can supply power to a heating element when the battery requires additional heat.
Such batteries are generally used downhole until they have run out of charge and they then have to be removed with the entire downhole arrangement on which they are provided. It would be desirable to extend the period of time that an energy storage device can be used downhole to reduce operational interruptions in which the entire downhole arrangement has to be retrieved, the energy storage device recharged or replaced and passed downhole again.